In cardiovascular surgery, as well as other medical applications, there is frequently a need to extend very thin (few millimeter diameter), long (30-150+ cm) and sterile catheters into thin-walled (e.g., 1-1.5 millimeter wall thickness) biological lumens, including blood vessels such as arteries and veins.
A number of vascular diseases, such as coronary artery disease and peripheral vascular disease, are caused by the build-up of atherosclerotic deposits (plaque) in the arteries, which limit blood flow to the tissues that are supplied by that particular artery. Disorders caused by occluded body vessels, including coronary artery disease (CAD) and peripheral artery disease (PAD) may be debilitating and life-threatening. Chronic Total Occlusion (CTO) can result in limb gangrene, requiring amputation, and may lead to other complications and eventually death. Increasingly, treatment of such blockages may include interventional procedures in which a guidewire is inserted into the diseased artery and threaded to the blocked region. There the blockage may be either expanded into a more open position, for example, by pressure from an inflated catheter balloon (e.g., balloon angioplasty), and/or the blocked region may be held open by a stent. Treatment of such blockages can also include using a catheter to surgically remove the plaque from the inside of the artery (e.g., an atherectomy).
When the artery is totally blocked by plaque, it is extremely difficult, and potentially dangerous to force the guidewire through the occlusion. An obstruction or plaque may be composed of relatively tough fibrous material, often including hard calcium deposits. Forcing a guidewire or catheter past such obstructions may cause the guidewire to puncture the walls of the vessel (e.g., artery) or cause it to enter the layers forming the artery, further damaging the tissue. Thus, there remains a need for guidewire positioning devices that can effectively traverse occluded vessels, and particularly chronically occluded vessels. Such devices would enable positioning of a guidewire and therefore enable positioning of stents and other devices, leading to improved patient outcomes and a reduction in patient morbidity and mortality.
Moreover, there is medical interest in equipping catheter-based cardiovascular catheters with sensors that can help direct atherectomy and other surgical procedures. For example, it would be useful to have sensors that can give the surgeon immediate visual feedback as to whether a particular tissue is diseased and/or how far away the cutting portion of a catheter is from the boundary of a particular blood vessel layer to minimize the risk of accidental damage. Conventional radiological imaging methods and ultrasound imaging systems have been attempted for such surgical procedures. However, neither ultrasound nor radiological imaging methods have enough resolution to help guide the operation of the catheter over the critical last fraction of a millimeter between the interior of a blood vessel and the exterior of the blood vessel. Moreover, standard radiological techniques cannot easily discriminate between healthy tissue and diseased tissue unless the tissue has become heavily calcified. Further, the components of an ultrasound system are generally too large to implement in small dimensions.
Optical Coherence Tomography (OCT) has been proposed as one technique that may be particularly helpful for imaging regions of tissue, including within a body lumen such as a blood vessel. At a basic level, OCT relies on the fact that light traveling from a source and scattering from more distant objects takes longer to travel back than light scattering from nearby objects. Due to the wave nature of light, very small timing differences caused by light signals traveling different distances on the micron scale can cause constructive or destructive interference with reference light signals. OCT systems measure the resulting interference to obtain an image of the target. Unfortunately, however it has thus far proven difficult to provide stable and reliable OCT systems for use in a catheter. A typical OCT system requires one or more interferometers to distinguish the signal from the applied light. In addition, most known OCT systems, when applied to catheters, include a fiber that is rotated (often at high rates) within the catheter in order to scan around a lumen. These systems typically require relatively high power operation, since the many components necessary for rotating and managing the OCT pathway (e.g., fiber) result in optical losses.
Thus, there is a need for efficient and robust OCT systems that are compatible with catheter applications and uses. Described herein are enhanced Optical Coherence Tomography (OCT) systems that that overcome many of the problems described above.
Referring to FIG. 1, a typical OCT device includes a target arm and a reference arm to generate a reference signal. In order to provide the interference reference signal, the OCT device will split an illuminating light signal from the source in two equal or unequal parts, send part of the illuminating light to the target of interest through one target optical “target arm” and send the other part of the illuminating light down a separate reference arm. Light from the separate reference arm reflects off of a mirror, and then returns and interferes with the scattered light that is returning from the target optical arm after bouncing off of the target. In a traditional OCT device, the reference arm length is engineered to be exactly the same length as the target arm so that the interference effect is maximized. The resulting interference between the two beams creates interference effects known as fringes that can be used to measure the relative reflectivity of various layers of the target. Using this information, an image of the object can be generated.
By contrast to the more established applications for OCT, cardiovascular catheters, which are intended for one-time use in blood vessel environments, must be of the highest level of sterility. To obtain such sterility, cardiovascular catheters are typically produced as low-cost disposable items that can be factory sterilized. During a medical procedure, such a catheter is typically removed from the factory sterile container. The proximal end of the catheter is connected to equipment needed to control the catheter (which in this case would also include the link to the OCT engine used to drive any OCT optical fiber in the catheter), and the distal tip is immediately inserted into the patient's body. The catheter is then discarded once the procedure is complete.
Producing low-cost disposable catheters can be difficult as a result of the need for precise reference arm matching and expensive optics. Thus, there is also a need for a low-cost OCT catheter.